I know this isn't the popular answer today, but: When I'm designing an app from scratch, I always use JSPs. When the logic is non-trivial, I create ordinary Java classes to do the grunt work that I call from the JSP. I've never understood the argument that you should use servlets because, as pure Java classes, they are more maintainable. A JSP can easily call a pure Java class, and of course an ordinary Java class is just as maintainable as any servlet. It's easier to format a page in a JSP because you can put all the markup in-line instead of having to write a bunch of println's. But the biggest advantage of JSPs is that you can just drop them in a directory and they are directly accessible: you don't need to mess with setting up relationships between the URL and the class file. Security is easily handled by having every JSP begin with a security check, which can be a single call statement, so there's no need to put security into a dispatch layer.
The only reason I can see to use a servlet is if you need a complex mapping between URLs and the resulting execution class. Like, if you want to examine the URL and then call one of many classes depending on session state or some such. Personally I've never wanted to do this, and apps I've seen that do do it tend to be hard to maintain because before you can even begin to make a change you have to figure out what code is really being executed.
